Using the standard technique [1]} involving application of Rouche's theorem, one can prove that the sequence of \(c_2(z^2)\)  zeros has the form \(\lbrace  -z_n \rbrace _{n \ge 0} \cup \lbrace  z_n\rbrace _{n \ge 0}\)  together with the asymptotic formulae
\( z_n = \frac{\pi n}{l} + \frac{1}{d \pi n} + O\Big (\frac{1}{n^3}\Big ), \quad n \in \mathbb {N}.\) 
